Add Attribute class

One more thing is to add Attribute, which _should not_ inherit from `Node`, so this would be enough

```js
class Attribute {
constructor (name, value, ns) {
this.nodeType = 2
this.name = name
this.value = value
this.namespaceURI = ns
}
}
```

I can PR with fixes and updates addressed in couple of issues, do you want such PR?